"Marge... sometimes the best defense is a good offense." - Homer Simpson
You need to not get hit by cold iron weapons. At the end of the day, that's the only real way to negate that vulnerability. There's ways to mitigate it somewhat (raising your AC with Barkskin and items, spells like Stoneskin or Heart of Earth to get better damage reduction), but you just need to not get hit.
Hiding is obvious, but you're stuck in a room and enemies are coming to you. So that's not really viable. What to do? Be offensive in a way that forces them to fight on your terms, where they can't bring those dangerous weapons to bear against you.
As a Druid, you have an animal companion. It does not have Cold Iron vulnerability. Use it to your advantage. Animals with Improved Trip can trip enemies and keep them from getting to you (Wolves can do this, at higher level Dire Wolves are quite good at it).
A grappling companion can grab someone and lock it down, keeping it from being able to attack you at all. Anything with Improved Grab is good for this (Leopard has it at your level, Brown Bears & Dire Lions are popular choices, but above your level).
Don't forget that you can heal and buff your companion. You want them to be tough and tie stuff up while you attack from a distance.
Druids are good summoners. Bring in some extra monsters to tie up attackers and keep them off you.
You've got Wild Shape, which opens up some similar possibilities to what an animal companion can do. Particularly, grappling.
That might seem counter-intuitive, but once you can turn into large size creatures you can grapple medium size heroes pretty easily. A grappled creature can only use natural and light weapons. No hindrance to you at all... big problem to the Fighter with the Cold Iron Longsword.
He can't use it while in a grapple. Your weakness doesn't matter at all if he can't use his weapon. Also works on spellcasters, since it's much harder to cast while a bear is sitting on you clawing your face.